Carbonate Sedimentology and Sequence Stratigraphy

download Carbonate Sedimentology and Sequence Stratigraphy

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Transcript of Carbonate Sedimentology and Sequence Stratigraphy

CARBONATE SEDIMENTOLOGY AND SEQUENCE STRATIGRAPHY

Wolfgang SchlagerVrije Universiteit/Faculty of Earth and Life Sciences Amsterdam, Netherlands

Copyright 2005 by SEPM (Society for Sedimentary Geology) Laura J. Crossey, Editor of Special Publications Concepts in Sedimentology and Paleontology No. 8

Information from the original printed Concepts in Sedimentology and Paleontology #8

This book is published with funds from the SEPM Foundation Inc., including the Bruce H. Harlton Publications fund made possible by Allan P. Benniston

ISBN 1-56576-116-2 2005 by SEPM (Society for Sedimentary Geology) 6128 East 38th Street #308 Tulsa, Oklahoma 74235, U.S.A.

Camera-ready by E. van Bentum, Amsterdam Printed in the United States of America

Information for this CD version of Concepts in Sedimentology and Paleontology #8

ISBN 978-1-56576-132-2 2007 by SEPM (Society for Sedimentary Geology) 6128 East 38th Street #308 Tulsa, Oklahoma 74235, U.S.A.

Camera-ready by E. van Bentum, Amsterdam Printed in the United States of America

To Hanneke Schlager-Vierstraete wife, mother and silent co-author

ContentsPreface 1 Essentials of neighboring disciplines Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Some principles of oceanography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Layering of the oceanic water column . . . . . . . . . . . . . . . . . . . . . . . Present surface circulation of the oceans . . . . . . . . . . . . . . . . . . . . . . Present deep circulation of the ocean . . . . . . . . . . . . . . . . . . . . . . . . Distribution of temperature, salinity, nutrients and light in the surface waters Essentials of carbonate mineralogy and chemistry . . . . . . . . . . . . . . . . . . . Ecology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Law of sigmoidal growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Life strategies of organisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Food chains, nutrient levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Life and light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Species diversity and environment . . . . . . . . . . . . . . . . . . . . . . . . . From oceanography , chemistry and biology to geology . . . . . . . . . . . . . 2 Principles of carbonate production Modes of marine carbonate precipitation . . . . . . . . . . . . . Abiotic marine carbonate precipitation . . . . . . . . . . . Biotically controlled precipitation . . . . . . . . . . . . . . Biotically induced precipitation . . . . . . . . . . . . . . . Precipitation modes in comparison . . . . . . . . . . . . . From precipitation modes to carbonate factories . . . . . . . . T factory . . . . . . . . . . . . . . . . . . . . . . . . . . . . C factory . . . . . . . . . . . . . . . . . . . . . . . . . . . . M factory . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sedimentation rates and growth potential of the three factories Carbonate-specic aspects of deposition and erosion . . . . . . Deposition source and sink . . . . . . . . . . . . . . . . Erosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mechanical erosion . . . . . . . . . . . . . . . . . . . . . . Chemical erosion . . . . . . . . . . . . . . . . . . . . . . . Bioerosion . . . . . . . . . . . . . . . . . . . . . . . . . . . Sea cliffs . . . . . . . . . . . . . . . . . . . . . . . . . . . . Carbonate rocks their description and classication . . . . . 3 Geometry of carbonate accumulations Basic trends in geometry of carbonate accumulations Localised accumulations . . . . . . . . . . . . . . Ramp . . . . . . . . . . . . . . . . . . . . . . . . . Slope, rise, basin oor . . . . . . . . . . . . . . . Slope curvature . . . . . . . . . . . . . . . . . . . Geometry of T, C and M factories . . . . . . . . . The empty bucket . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii 1 1 1 1 1 3 3 6 8 8 9 9 10 10 11 13 13 13 14 20 20 21 21 24 24 24 25 25 25 28 28 31 34 35 39 39 40 43 43 45 47 51

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CONTENTS 4 Carbonate facies models Unda, clino and fondo environments . . . . . . . Facies patterns from ramp to rimmed platform Facies belts of the T factory . . . . . . . . . . . . . Discription of the standard facies belts . . . Facies belts of the C factory . . . . . . . . . . . . Facies belts of the M factory . . . . . . . . . . . . Carbonate facies of epeiric seas . . . . . . . . . . Stability of facies patterns . . . . . . . . . . . . . Bias in the facies record . . . . . . . . . . . . . . . Terrestrial exposure . . . . . . . . . . . . . . . . Megabreccias . . . . . . . . . . . . . . . . . . . . . Environmental messages from organisms . . . . Siliciclastics and evaporites in carbonate facies .

v 55 55 56 57 57 63 63 63 67 67 67 68 68 69 73 73 74 79 80 83 83 83 83 86 89 90 91 91 94 96 96 98 99 100 100 101 101 105 105 105 105 105 107 108 109 110 114 115 115 116 116 116 119 121 121 126 126 130 133 133 137 140 144

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Rhythms and events in carbonate stratigraphy Autocycles . . . . . . . . . . . . . . . . . . . . . . . Orbital rhythms . . . . . . . . . . . . . . . . . . . . Long oscillations in the ocean-atmosphere system Biotic evolution . . . . . . . . . . . . . . . . . . . .

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Fundamentals of sequence stratigraphy Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Principles and denitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sequence and sequence boundary . . . . . . . . . . . . . . . . . . . . . . . . Systems tracts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Stratigraphic time lines and seismic reections . . . . . . . . . . . . . . . . . Unconformities in outcrop and seismic data . . . . . . . . . . . . . . . . . . . Parasequence and simple sequence . . . . . . . . . . . . . . . . . . . . . . . . Sea level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Accommodation and sediment supply - a dual control of stratigraphic sequences Orders versus fractals in the sequence record . . . . . . . . . . . . . . . . . . . . . Critique of the concept of orders in the standard model . . . . . . . . . . . . Fractals - an alternative to orders in sequence stratigraphy . . . . . . . . . . Support for the fractal nature of sequences . . . . . . . . . . . . . . . . . . . Fractals and the impression of ordered hierarchy . . . . . . . . . . . . . . . . Purpose and scope of the fractal model . . . . . . . . . . . . . . . . . . . . . Origin of scale-invariant fractal pattern . . . . . . . . . . . . . . . . . . . . . Origin of sequences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Sequence stratigraphy of the T factory Carbonate factories and the principle of depositional bias . . . . . . . . . . . . . . . . . . . T factory key attributes for sequence stratigraphy . . . . . . . . . . . . . . . . . . . . . . T factory - sequence anatomy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Importance of platform rims . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Systems tracts in T carbonates and their control by accommodation and production Sea-level movements deduced from seismic images of carbonate platforms . . . . . Shoal-water facies of T-factory systems tracts . . . . . . . . . . . . . . . . . . . . . . . . . . Systems-tract facies of modern Bahamas and Florida . . . . . . . . . . . . . . . . . . . The testimony of ancient systems tracts . . . . . . . . . . . . . . . . . . . . . . . . . . Ecologic reef, geologic reef, seismic reef . . . . . . . . . . . . . . . . . . . . . . . . . . Rules of thumb on systems tracts and facies . . . . . . . . . . . . . . . . . . . . . . . . T sequences in deeper-water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Periplatform environment - part of the platform system . . . . . . . . . . . . . . . . . Highstand shedding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Megabreccias and sea level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bounding surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sequence boundary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Transgressive surface and maximum ooding surface . . . . . . . . . . . . . . . . . . Pseudo-unconformities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Siliciclastics and evaporites in carbonate sequences . . . . . . . . . . . . . . . . . . . . . . . Two Neogene case studies in carbonate sequence stratigraphy . . . . . . . . . . . . . . . . Northwestern Bahamas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Late Miocene of Mallorca . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The sequence stratigraphy of gradual change . . . . . . . . . . . . . . . . . . . . . . . . . . High-resolution sequence stratigraphy of carbonates . . . . . . . . . . . . . . . . . . . . . .

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CONTENTS 147 147 147 147 147 149 150 150 152 152 152 157 161 161 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165 165 165 165 167 168 168 168 169 169 171 171 172 173 174 175 194 198

8 Sequence stratigraphy of C and M factories Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C factory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sequence facies and bounding surface . . . . . . . . . . . . . . . . . Geometry of systems tracts . . . . . . . . . . . . . . . . . . . . . . . Highstand shedding or lowstand shedding? . . . . . . . . . . . . . Sequence stratigraphy of the C factory in deep-water environments M factory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sequence anatomy and bounding surfaces . . . . . . . . . . . . . . . 9 Looking back, moving forward Appendices A Fractals B Introduction to modeling programs STRATA and CARBONATE 3D program STRATA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Diffusion equation for sediment dispersal . . . . . . . . . . . . . . Carbonate production functions . . . . . . . . . . . . . . . . . . . . CARBONATE 3D . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Carbonate production . . . . . . . . . . . . . . . . . . . . . . . . . Sediment dispersal . . . . . . . . . . . . . . . . . . . . . . . . . . . Additional options . . . . . . . . . . . . . . . . . . . . . . . . . . . C Principles of reection seismology Origin of seismic reections . . . . . . . . . . . . . . . . Synthetic seismic traces and seismic models of outcrops Seismic resolution . . . . . . . . . . . . . . . . . . . . . . Seismic attributes . . . . . . . . . . . . . . . . . . . . . . References Subject Index First author index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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PrefaceSedimentology and stratigraphy are neighbors yet distinctly separate entities within the earth sciences. Put in a nutshell, sedimentology searches for the common traits of sedimentary rocks regardless of age as it reconstructs environments and processes of deposition and erosion from the sediment record. Stratigraphy, by contrast, concentrates on changes with time, on measuring time and correlating coeval events. Sequence stratigraphy straddles the boundary between the two elds. It is a sedimentologic concept as it uses depositional anatomy to reconstruct environments and lateral facies change, and it is part of stratigraphy as it studies the vertical succession of sedimentary rocks and their succession and correlation. This expose, dedicated to carbonate rocks, approaches sequence stratigraphy from its sedimentologic background. Carbonate sedimentation, in contrast to siliciclastic sedimentation, is largely governed by chemistry and biota of the ocean and thus intimately tied to the ocean environment. Therefore, the presentation starts with essentials of physical and chemical oceanography and biology. It then proceeds to principles of marine carbonate production (and erosion) and the geometry of carbonate accumulations, using the concept of carbonate production systems, or factories, to illustrate the variations among carbonate rocks. Armed with the knowledge on production and accumulation, the text turns to carbonate facies; the sedimentologic part closes with an overview of the rhythms and events governing carbonate deposition in time and space. Chapters 6 through 8 deal with sequence stratigraphy. This part starts with an overview of the standard model of sequence stratigraphy and then develops carbonate sequence stratigraphy on the basis of processes and principles presented in the sedimentologic part and using the three major carbonate factories as a template for discussion. The book attempts to make progress by combining different specialties and different lines of reasoning, and by searching for principles underlying the bewildering diversity of carbonate rocks. I think the expose provides enough general background, in introductory chapters and appendices, to be easily digestible for sedimentologists and stratigraphers as well as earth scientists at large. The text does appeal to the readers willingness to engage in scientic discussion, however. It is not a cook book presenting recipes. The book bears the name of a single author but many have supported me in the endeavor. Gregor Eberli (University of Miami), Mitch Harris (Chevron, San Ramon), Adrian Immenhauser (Vrije Universiteit, Amsterdam) and Rick Sarg (Exxon, Houston) reviewed the entire manuscript, Eberli and Harris did so as ofcial reviewers for SEPM. Advice on special topics was offered by Giovanni Bertotti (Vrije Universiteit, Amsterdam) on tectonics, Bruce Fouke (University of Illinois, Urbana) on sea cliffs, Henk Kooi (Vrije Universiteit, Amsterdam) on slope stability, and Georg Warrlich (Shell, Rijswijk) on computer modeling. Important data were contributed by Flavio Anselmetti (ETH, Zrich), Hemmo Bosscher (Shell, Rijswijk), Annette George (University of Western Australia, Crawley), Robert Ginsburg (University of Miami), Mitch Harris (Chevron, San Ramon), Lisa Hinnov (Johns Hopkins University, Baltimore), David Hunt (Norsk Hydro, Bergen), Adrian Immenhauser (Vrije Universiteit, Amsterdam), Alexandra Isern (National Science Foundation, Washington), Noel P. James (Queens University, Kingston), Bernd Kaufmann (University of Tbingen), Lorenz Keim (Geological Survey, Bolzano), Jeroen Kenter (Vrije Universiteit, Amsterdam), Don McNeill (University of Miami), Sam Purkis (Nova University, Dania) Franco Russo (University of Cosenza), Orson van de Plassche (Vrije Universiteit, Amsterdam), Valentina Zampetti (Vrije Universiteit, Amsterdam). Many of the ideas expressed in the book developed in the stimulating environments of Miamis Rosenstiel School of Marine and Atmospheric Science, particularly the Comparative Sedimentology Laboratory founded by Robert Ginsburg, and the Sedimentology Section of the Earth Sciences Faculty at the Vrije Universiteit Amsterdam. At both institutions, Industrial Associates Programs offered valuable feedback from geologists in industry. Another crucial sounding board were the participants of short courses I taught on the subject in North America, Europe and Southeast Asia between 1991 and 2003. I was very fortunate with the publication process. Elisabeth van Bentum was my partner in the production of the manuscript. Her energy and skills in drafting and camera-ready editing made for smooth sailing all along. SEPM, with Howard Harper at the helm, Laura Crossey as Editor-in-Chief and Kris Farnsworth as publications coordinator supported and encouraged me throughout. Wolfgang Schlager, Amsterdam

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PREFACE

C HAPTER 1

Essentials of neighboring disciplinesINTRODUCTION Unraveling Earth history is a core business of geology. However, understanding cause and effect of past events requires input from other disciplines that can study processes directly and do not have to reconstruct them from incomplete historic records. This introductory chapter summarizes a very limited number of concepts from neighboring disciplines that are relevant in this respect. The list is woefully incomplete. I did not try to simply cover the most important concepts; rather, I selected those that are highly relevant for the topic of this book yet not sufciently important for geology at large to be routinely covered by introductory texts or courses in geology. SOME PRINCIPLES OF OCEANOGRAPHY Layering of the oceanic water column The density of ocean water varies as a function of temperature and salinity. Under the inuence of the Earths gravity eld denser water sinks and lighter water rises and this creates a density-stratied, layered ocean (Weyl, 1970; Open University, 1989a, 1989b). In rst approximation, three layers may be distinguished: (Fig. 1.1) A surface layer, where waves and currents are sufciently strong to preclude the formation of permanent density gradients; the surface layer is thus well mixed and in equilibrium with the atmosphere with regard to oxygen and other chemical agents. The thermocline, where density increases steadily downwards and vertical mixing is greatly inhibited. The oceanic deep water, where density slowly increases downward but the vertical gradient is so low that it is easily disturbed by horizontal ow. Water in all layers circulates. Circulation in the surface layer is driven by winds in the atmosphere. The deep layer presently forms one world-wide circulation system that moves slowly under the inuence of minor density gradients. Surface layer and deep layers are generally well separated but they communicate in certain, well dened areas. In the northern North Atlantic and around Antarctica, surface water becomes so dense that it sinks and joins the deep-water body. Conversely, along the west-facing coastlines of Africa and the Americas the surface layer is driven offshore by 1 winds and the Coriolis force such that thermocline water and deep water well up. Deep water also rises to the surface in the Antarctic Current because of the currents extremely low density gradient. More gentle upwelling occurs in the equatorial Pacic where the opposing directions of the Coriolis force of the northern and southern hemisphere drive the surface layer away from the equator. See Fig. 1.2 for cartoons of upwelling mechanisms. An important principle in all interactions between atmosphere and ocean is Ekman transport: wind blowing over the ocean induces a current in the surface layer approximately at right angles to the wind. This current moves to the right of the wind in the northern hemisphere and to the left in the southern hemisphere. When using the modern oceans as models for the geologic past, we should keep in mind that currently temperature has a greater effect on the density stratication than salinity. The main reason for the dominance of temperature are the ice caps of the poles. At times in the past when little ice was present, salinity seems to have been the dominant factor in density stratication (e.g. Hay, 1988). Present surface circulation of the oceans The surface circulation greatly inuences the distribution of temperature, salinity and nutrients in the surface layer and those properties, in turn, are important controls on carbonate precipitation and deposition. We shall examine the present circulation to derive guidelines for interpreting the past. For more detail, see Weyl (1970), Broecker and Peng (1982), Open University (1989a, 1989b), Emiliani (1992, p. 254-309). The chain of cause and effect between atmosphere and surface ocean starts with the Sun. It heats the equatorial belt of the Earth much more than the polar regions. As a consequence, the air over a stationary Earth would rise over the equator, ow poleward at high altitude, cool, sink and ow back towards the equator near the Earths surface. This simple circulation model immediately calls for two essential modications: the Coriolis force of the rotating Earth deects the ow of air. Air starting to move from pole to equator will be deected to move from east to west as the Coriolis effect turns it to the right in the northern and to the left in the southern hemisphere. Conversely, air starting to ow poleward from the equator will turn to ow eastward.

2

WOLFGANG SCHLAGER

Fig. 1.1. Upper water column of the tropical and temperate parts of the ocean. A) Principal layering of the water column. B) Temperature prole (solid line) indicates low gradients in the mixed layer, extremely high gradients in the uppermost thermocline, high gradients in the lower thermocline and low gradients in the oceanic deep water. C) Characteristic proles of the concentration of dissolved oxgen and phosphate (a nutrient). These proles reect the interplay of the density structure of the water, gas exchange with the atmosphere and organic growth and decay. Oxygen and nutrient levels are crucial for subdividing the carbonate producing environments. See text for details.

a single circulation cell per hemisphere cannot be sustained because of rapid cooling in the upper atmosphere and the speed of the Earths rotation. The single cell breaks up into three cells where air rises and sinks as shown in Fig. 1.3A. The three cells induce three major windelds per hemisphere easterly trade winds in the low latitudes, westerlies in the temperate belt and easterlies again in the subpolar regions.

The three wind elds between pole and equator induce surface currents in the ocean. If the Earth were entirely covered by water, we would see globe-circling currents owing from east to west near the poles, from west to east in the temperate zone of westerly winds and from east to west again in the subtropical belt of trade winds. Around the equator where winds are weak and irregular, surface currents will also be weak. The present distribution of land masses severely obstructs the development of globe-circling currents. Pacic and Atlantic both extend from the Arctic to the Antarctic but are bounded in the west and east by continents or archipelagos. The Indian Ocean extends over a little more than one hemisphere in the same fashion. Thus, is behooves us to consider a model ocean stretching from pole to pole but bounded in the east and west by land (Fig. 1.3B; Weyl, 1970). In this model, the latitudinal currents induced by the wind elds are deected to form circular currents called gyres. The largest ones, the subtropical gyres, develop in both hemispheres as water ows westward un-

der the trades and returns to the east under the westerlies. The smaller subpolar gyres circle counterclockwise under the combined effect of westerlies in the temperate zone and easterlies in the subpolar region. In the equatorial region, our model ocean shows two narrow, counterclockwise gyres as part of the water piled up against the western border of the ocean returns eastward in the equatorial calm zone, the doldrums. Fig. 1.4 depicts the real surface circulation of the world ocean. It shows most features predicted Weyls (1970) model ocean. The most signicant departure is around Antarctica where north-south land barriers are absent so that a globecircling current, the Antarctic Current, could develop. Furthermore, the narrow equatorial gyres and the counter current separating them are weakly developed in the Atlantic where the equatorial system points WNW-ESE because of the peculiar shape of the continental borders, and in the Indian Ocean, where India deforms the northern subtropical gyre and monsoonal circulation disturbs the trade-wind system. Monsoons are winds that reverse direction in opposite seasons because of the different heating and cooling of the air over land and sea. Air over a large land mass at temperate latitudes becomes cool and dense in winter; a highpressure cell forms and wind streams out from it. Conversely, air in the summer is hot and light; a cell of low pressure develops and sucks in air from adjacent oceans where

CHAPTER 1: ESSENTIALS OF NEIGHBORING DISCIPLINESA) Open-ocean Coriolis effect upwelling B)

3

wind-driven upwelling

Present deep circulation of the ocean The principal driver of present deep-water circulation is the formation of dense cold water in the northern North Atlantic and on the shelves of Antarctica. This water sinks and provides the deep and bottom water for all major ocean basins. The rst-order pattern is that most deep water forms in the Atlantic and ows through the Indian Ocean to the North Pacic whence it returns to the source areas via upwelling and surface circulation. (Fig. 1.5; Broecker and Peng, 1982; Open University, 1989b). This ow pattern forces the three principal oceans to constantly exchange surface water and deep water at a large scale. The effects of this exchange on the three basins are very different, though, and this leads to differences in water chemistry that are relevant for carbonate sedimentation. The Atlantic donates deep water and receives surface water, the Pacic receives deep water and donates surface water, and the situation of the Indian Ocean is intermediate. As nutrient concentrations are generally low in surface water and high in deep water, the Atlantic is depleted in nutrients compared to the Pacic. Fig. 1.6 depicts this principle of basin-basin fractionation of dissolved substances. Another side effect of deep circulation concerns carbonate dissolution at depth. The deep water in the Atlantic is relatively young, thus still rich in oxygen (from the time spent at the sea surface) and low in carbon dioxide because the water has not had time to oxidize much organic matter. Conversely, Pacic deep water is old, low in oxygen and high in carbon dioxide because of the long time spent oxidizing organic matter without an opportunity to take up oxygen from the atmosphere. The low CO2 content of the deep Atlantic leads to high carbonate saturation and a low position of the carbonate compensation depth (CCD) the level where the rate of carbonate sedimentation equals the rate of carbonate dissolution. The situation is reversed in the Pacic. There, the deep water is old, rich in CO2 and thus largely undersaturated with respect to calcium carbonate. This increases the rate of dissolution and raises the carbonate compensation depth. The world map in Fig. 1.7 clearly shows the differences between Atlantic and Pacic. Distribution of temperature, salinity, nutrients and light in the surface waters Temperature, salinity and nutrients in the surface layer of the sea are important controls on carbonate production. Their distribution is mainly governed by the latitudinal differences of solar radiation and by the patterns of surface circulation. The rst-order trend is a decrease in temperature with increasing latitude and salinity maxima in the horse latitudes that correspond to the desert belts on land. From the horse latitudes, salinity decreases pole-ward and towards the equator where high rainfall dilutes the surface water. The effects of gyre circulation are more varied as outlined below.

Meteorological equatorSoutheast trades Northeast trades

offshore wind

C)

Coriolis effect transport (shown for northern hemisphere)

D)

obstruction upwelling (plan view)

Fig. 1.2. Most efcient processes causing upwelling. A) Ekman transport along the equator drives the surface layers away in opposite directions. B) Wind blowing offshore drives the surface layer away from the coastal zone and exposes deeper water layers. (Ekman transport starts at high angles to the wind but quickly turns into wind-parallel motion because of space constraints). C) Ekman transport induced by coast-parallel winds or surface currents drives the surface layer offshore and exposes deeper layers (situation shown is for the northern hemisphere). D) Local upwelling where shore-parallel current is impeded by a promontory in the coastline. After Pipkin et al. (1987), modied.

the reversing monsoon winds may induce reversing surface currents. Currently, the strongest monsoons develop between southern Asia and the Indian Ocean. Fig. 1.4 shows that the present world ocean shows features of Weyls laterally bounded gyre ocean as well as a globe-circling current. Paleoceanographic studies indicate that the changing positions of continents, archipelagos and island arcs of the geologic past also changed the coeval circulation patterns. It seems, however, that with proper adjustments, the concepts of oceanic gyres and globe-circling currents are extremely useful models for the reconstruction of the past, including the reconstruction of shoal-water carbonate deposition.

current

4A)atmosphere

WOLFGANG SCHLAGERB)atmosphere

ocean surface (gyres)

polar frontwesterlies horse latitudes

subarctic

N subtropical

trade winds doldrums

N equatorialS equatorial

Fig. 1.3. Weyls (1970) model of surface circulation in the ocean. A) The atmosphere has three circulation cells per hemisphere. In the northern hemisphere of a non-rotating Earth, these cells would induce winds at the surface of the northern hemisphere that blew northward between equator and horse latitudes, southward between horse latitudes and polar front, and northward again in the polar region. The Coriolis force deects these winds to the right, thus creating the wind elds we currently observe. B) The wind elds depicted in Fig. A shear over the surface layer of the ocean and induce currents there. The surface currents deviate to the right of the wind vector in the northern hemisphere because of Ekman transport caused by the Coriolis force. The results are west-owing currents under the trade winds, east-owing currents under the westerlies, and west-owing currents in the polar regions. In modern ocean basins, N-S running coasts block most of these currents and deect them to form large closed circulation loops called gyres.

60

30

0

30

60

Fig. 1.4. Surface circulation in the modern oceans. After Strahler (1971), modied. The principal features of Weyls model are present, albeit with some distortions due to the shape of the ocean basins. In the south, around Antarctica, the gyre-circulation is replaced by the globe-circling Antarctic Current.

CHAPTER 1: ESSENTIALS OF NEIGHBORING DISCIPLINESAtlanticSource

5B) anti-estuarine circulation

PacificEurope-Asia

A) estuarine circulation

Americas

Africa

Americas

oxygen

Indian

carbon dioxide nutrients

Source

Fig. 1.5. Present deep-water circulation in the ocean. After Broecker and Peng (1982, Fig. 1-12), modied. Deepwater (black arrows) originates in the northern North Atlantic and on the shelves of Antarctica and ows through all three oceans. Return ow occurs by surface circulation (gray). All pathways are extremely simplied. Deep-water rises to the surface in upwelling areas, mainly on the west sides of continents (dots) and in the Antarctic current.

Fig. 1.6. Basin-basin fractionation of nutrients, oxygen and carbon dioxide (based on Berger and Winterer, 1974; Broecker and Peng, 1982). A) Estuarine circulation. This ocean basin circulates like a river estuary. It donates surface water to, and receives deep water from the world ocean. Consequently, its deep water is old, rich in carbon dioxide and nutrients, and low in oxygen. Modern example: Pacic. B) Anti-estuarine circulation. This ocean basin circulates like a hypersaline lagoon. It donates deep water and receives surface water. Its deep water is young, rich in oxygen and depleted in carbon dioxide and nutrients. Modern example: Atlantic.

3 3.54

3.5 4

4

443.5

4

3.5

3

3.5 - 4.0 km

4.0 - 4.5 km

4.5 - 5.0 km

5.0 - 5.5 km

> 5.5 km

Fig. 1.7. Carbonate compensation depth (CCD) in the recent oceans, determined from the carbonate content of pelagic sediments. The CCD forms a surface with considerable relief. This surface is relatively deep in the Atlantic; relatively shallow in the Pacic and lies at intermediate depths in the Indian Ocean. The difference among ocean basins is caused by basin-basin fractionation. In all three oceans the CCD shoals towards the ocean margins as a result of high organic productivity and concomitant production of CO2 . Gentle equatorial upwelling in the Pacic increases planktonic carbonate production and thus causes a depression of the CCD. After Berger and Winterer (1974), modied.

The subtropical gyres are of special importance for carbonate sedimentation because of their effect on nutrient concentrations and surface productivity (Fig. 1.8). The basic rule is that areas of upwelling are productivity maxima, ar-

eas with old surface waters are productivity minima. The rotation in the gyres is such that the Coriolis force deects the water towards the center of the gyre, creating a zone of convergence, lled with old, nutrient-depleted water of

6

WOLFGANG SCHLAGER

25 - 40

40 - 90

90 - 125

125 - 500 gCm-1.yr-1

Fig. 1.8. Organic productivity in the surface layer of the ocean, calculated from phosphate distribution, latitude and distance from shore. Typical are minima (submarine deserts in the subtropical gyres and maxima in the coastal upwelling zones, particularly along the west-facing coasts. High productivity also occurs in the globe-circling Antarctic current where water of all depths mixes easily because density gradients are very low. Carbonate production is strongly inuenced by nutrient concentration (see chapter 2). After Berger (1989), modied.

high salinity because the gyre centers lie in the arid horse latitudes. The centers of the gyres represent warm, nutrientdepleted deserts of the ocean. The eastern peripheries of the subtropical gyres represent the other extreme. The ow pattern is such that wind and Coriolis force drive the surface water away from the continent. Upwelling of cool, nutrient-rich water is the result. At the western periphery of the gyres, upwelling is weaker as wind drift and Coriolis force oppose one another. In the equatorial belt, the opposed direction of the Coriolis force in the two hemispheres leads to surface divergence and gentle upwelling with moderately high nutrient levels. Upwelling and high rainfall lower the salinities. Light is the basis for photosynthesis, i.e. organic growth that relies on dissolved nutrients in the water and energy from the Sun. Organic growth by photosynthesis is the begin of the food chain of the ocean. Moreover, much carbonate sediment is formed as a byproduct of photosynthesis. The decrease of light with water depth follows a simple exponential function

ESSENTIALS OF CARBONATE MINERALOGY AND CHEMISTRY The material for carbonate sedimentation is extracted from the dissolved load of the sea; the volume of carbonate rocks derived from erosion of older rocks is very small. The precipitation reactions can be summarized as Ca2+ + 2HCO3 CO2 + H2 O + CaCO3

Aquatic precipitation proceeds along biotic and abiotic pathways that are discussed further in chapter 2. Precipitation, preservation and alteration of the carbonate rocks are strongly inuenced by their mineralogy. Three minerals appear in signicant amounts: aragonite, calcite and dolomite (Fig. 1.10). In practice, calcite is further subdivided into rather pure calcite (also called low-magnesium calcite) and magnesian calcite (or high-magnesium calcite). Magnesian calcites are generally dened as calcites with more than 4 mol% of CaCO3 replaced by MgCO3 kz Iz = I0 .e (J.A.D. Dickson in Tucker and Wright, 1990). This boundwhere I0 and Iz are the irradiance at sea level and depth ary is justied by the observed changes in solubility (Fig. z, respectively, and k is an attentuation coefcient that de- 1.11). Up to 4mol% MgCO3 , magnesium does not seem to pends on the turbidity of the water. For instance, k is large signicantly inuence calcite solubility. Therefore it is pracin areas of high suspended sediment load or high plank- tical to draw the boundary at the 4% level. One should be ton productivity; k is small in clear, low-productivity waters aware, however, that there is a continuous range of magnesium contents in calcite from 0 to over 30 mol% MgCO3 . such as at the centers of the subtropical gyres (Fig. 1.9).

CHAPTER 1: ESSENTIALS OF NEIGHBORING DISCIPLINES

7

50

0

Jerlov depth (m) type 110 90 50 70 55 30 1 1A 1B 2 30 3000 Km

Scale at the Equator.

Fig. 1.9. Crude estimate of the thickness of the euphotic zone based on the optical water types of Jerlov (1976). The euphotic zone is thickest in the subtropical gyres and thins towards higher latitudes. The euphotic zone also thins in the zone of equatorial upwelling and near tropical river discharge (Amazone, Indonesia). Modied after F. Vijn and H. Bosscher (written comm.)

Calcite, aragonite and dolomite differ considerably in solubility and the differences are sedimentologically very important. In sea water (and also in many pore waters) the ranking in terms of solubility is aragonite > calcite > dolomite. The solubility of magnesian calcite depends on the magnesium content as explained above. From 4 mol% onwards, solubility steadily increases with Mg content (Fig. 1.11). Magnesian calcite of >12mol% MgCO3 is more soluble than aragonite, magnesian calcite of lower MgCO3 content is less soluble than aragonite (Morse and Mackenzie, 1990; Morse, 2004). In view of this solubility ranking, it is surprising to see that carbonate sediments in the tropical marine environment consist largely of aragonite and magnesian calcite with mi-

nor fractions of calcite; primary dolomite forms only in special environments. Cool-water carbonates are richer in calcite but still contain mostly aragonite and magnesian calcite. The reason for this paradox is that most precipitation is controlled or induced by organisms and most abiotic reactions are inhibited by reaction kinetics. Thus, thermodynamics is a poor predictor of carbonate precipitation reactions in marine environments. Nearly all marine surface waters are supersaturated with respect to calcite and dolomite but the appropriate precipitation reactions are blocked in various ways. The abiotic reactions that ultimately do occur, such as the formation of brous cements in marine environments, produce aragonite or magnesian calcite rather than the thermodynamically expected minerals calcite and dolomite.

mineralAragonite Calcite Magnesian Calcite

isomorphic substitutionrhombic rhombohedral rhombohedral

density g.cm-3 2.94 2.72

Ca CO3 Ca CO3 Ca CO3with 5 - 44% Ca replaced by Mg

Sr, Na Mg, Sr, Na

Dolomite

Ca Mg (CO3)2 rhombohedral

Sr, Na, Fe

2.89

Fig. 1.10. Common carbonate minerals, their chemical composition, crystallography, and density.

8

WOLFGANG SCHLAGERkeep up rapid creation of niche catch up

7.0 7.2

calcite

magnesian calcite

7.2

log IAPmagnesian calcite

rate of growth

7.2

7.2 8.0 8.2 8.4 8.6 8.8

start up time

Fig. 1.12. Sigmoidal growth curves a common pattern in carbonate production. Populations of organisms respond to the opening up of new living space in three steps: rst, growth lags behind the creation of living space, second, population growth exceeds the rate of change in space, nally, population growth is limited by the rate of growth in living space. A sigmoidal growth curve results from this three-phase growth. Most carbonate systems follow this pattern and the terms on the right are widely used for the three stages of sigmoidal carbonate growth, (e.g. Neumann and Macintyre, 1985.)

0

10 20 mole% MgCO3

30

tion. The amount of dolomite formed in the depositional environment is minor; where dolomite is formed in these settings, the process is often induced by microbes (see Machel, 2004, for review). ECOLOGY Marine carbonate precipitation and deposition are closely related to life in the ocean. Ecology, the study of the relationship of organisms and their environment, provides a number of very useful concepts for carbonate sedimentologists. A selection of them is presented below. Law of sigmoidal growth Growth of a population of organisms follows a sigmoidal curve consisting of a slow starting phase, a phase of rapid growth and a nal phase of slow down where the population approaches a steady state in equilibrium with the carrying capacity of the living space (Fig. 1.12). The logistic equation is a mathematical expression that produces sigmoidal growth curves (Fig. 1.13). The equation describes the interaction between the intrinsic growth rate of a population and the limits set by the size of the living space. At early stages of growth, living space is virtually unlimited and the growth rate is given by dN/dt = rN where N is the number of individuals in the population, t is time, and r is the intrinsic reproduction rate of an individual. This relationship leads to exponential growth. The

Fig. 1.11. Solubility of calcites as a function of MgCO3 content. Curve represents the general trend of several experiments indicated by different symbols. Solubility, expressed as ion activity product, increases steadily from about 4 mol% MgCO3 upward. Dashed line: generally accepted boundary of calcite and magnesian calcite. After Mackenzie et al. (1983), modied.

Aragonite and magnesian calcite are metastable in most marine environments, i.e. they precipitate and exist for considerable time because of the effects of reaction kinetics mentioned above. However, diagenesis replaces aragonite and magnesian calcite by calcite and dolomite on geologic time scales. Replacement proceeds by dissolution-precipitation reactions rather than solid-state inversion. This implies that porosity may be created or destroyed and chemical signals reset in different diagenetic environments. Most magnesian calcite and aragonite disappear in less than a million years and are replaced by calcite and dolomite. Whether there is also a (much slower) conversion from calcite to dolomite remains debatable because of the likely overprint by the evolution of ocean chemistry ( Morse and Mackenzie, 1990, p. 548; Veizer and Mackenzie, 2004 ). The big bodies of dolomite in the stratigraphic record seem to have replaced limestones relatively early during burial diagenesis, i.e. millions to tens of millions of years after deposi-

CHAPTER 1: ESSENTIALS OF NEIGHBORING DISCIPLINES

9

dN = rN dtdN N = rN 1dt K

K

(

)

N

0

Time (t)

grow more and reproduce less or vice versa (Townsend et al., 2003). In a very inuential book, MacArthur and Wilson (1967) introduced two extreme life strategies that correspond to these alternatives. There are organisms, such as weeds, that quickly appear in newly opened habitats, multiply rapidly, are short-lived and relatively small. They are called r-strategists because they spend most of their life in the r-dominated, nearly exponential part of the population growth curve in Fig. 1.13. At the other end of the spectrum are organisms, such as large trees in a forest, that live in environments of intense competition for limited resources. These organisms grow fast, live long, and reproduce slowly and relatively late in their life; they are called K-strategists as most of their life is spent in the K-dominated part of the growth curve. Food chains, nutrient levels Wherever life exists on Earth, the organisms have developed a network of interactions, connecting plant with grazer, predator with prey etc. (Townsend et al., 2003). All interactions in a given ecosystem constitute the food chain. This chain normally begins with photosynthesis, the growth of organic tissue in green plants using energy from the Sun and dissolved inorganic chemical substances. Alternatively, the food chain may start with chemosynthesis, where bacteria grow by oxidizing methane, hydrogen sulde or other substances. Marine carbonate production nearly always depends on photosynthesis as a starting point. The organisms at the starting point of the food chain are called autotrophs (literally: self-feeders); organisms further down the food chain depend on other organisms for food and are called heterotrophs. The rate of photosynthetic production, the primary productivity, in the marine environment depends on the light intensity and the concentration of dissolved nutrients, such as phosphorous, nitrogen or carbon. Fig. 1.8 shows the rstorder patterns of nutrient concentration in the world ocean. High-productivity zones rim the major ocean basins, there is an equatorial belt of elevated productivity and another globe-circling belt around Antarctica. Finally, production minima characterize the central parts of the ocean basins, particularly in the subtropics. Comparison of Figs 1.8 and 1.4 reveals that the productivity patterns largely reect the surface circulation pattern of the ocean high productivity occurs where upwelling of deeper water brings nutrients to the surface, production minima occur where the water has remained at the surface for a long time. More specically, the high-productivity rings around the ocean basins coincide with zones of coastal upwelling, the productive equatorial belt marks the gentle upwelling generated by the opposing effects of the Coriolis force in the northern and southern hemisphere, and the belt around Antarctica is caused by upwelling within the Antarctic Current. Finally, the productivity minima lie in the subtropical gyres where old, nutrientdepleted, saline surface water is swept together by wind and

Fig. 1.13. Exponential growth (bold) and growth governed by the logistic equation (dashed). In natural systems, an initial phase of exponential growth usually gives way to decelerating growth that goes to zero as population size approaches the limits of space or other resources. The logistic equation produces sigmoidal growth by applying a rapidly increasing correction to the exponential growth term. This correction consists of the ratio of population size, N, and the carrying capacity of the environment, K. After Townsend et al. (2003), modied.

limitations of growth imposed by the nite size of the living space can be added to this equation by introducing the term N/K, the ratio of population size, N, and carrying capacity of the living space, K. This yields the logistic equation dN/dt = rN [1 ( N/K )] Solutions to this equation are sigmoidal curves as in Fig. 1.12. Initial growth rates are very nearly exponential because N/K, the growth reduction imposed by the limited size of the living space, is negligibly small. As N/K increases, growth rates progressively deviate from the exponential trend; they approach zero, i.e. constant population size, as N/K approaches unity. The logistic equation is one of the simplest equations producing sigmoidal curves. It was invented in the 19th century for modelling the growth of populations of living beings but it applies to abiotic systems, too, if there is some sort of competition among the individual components of the population. In carbonate sedimentology, the equation is useful for describing growth of carbonate production systems as well as purely mechanical accumulations. Life strategies of organisms The logistic equation and its sigmoidal growth curve lead to another important topic in biotic carbonate production the different life strategies of organisms. As a rule, energy and other resources available to an organism are limited and need to be partitioned between growth and reproduction. In other words: organisms need to decide to either

10

WOLFGANG SCHLAGER

oligotrophic

0.01A) dominant

nutrient gradient (mg Chl/m3) mesotrophic eutrophic 0.1 1.0

hypertrophic 10

special signicance in the continuum from light to dark the base of the euphotic zone and the base of the zone of light saturation (Fig. 1.15).

benthosB) primary

coral

coral-algae

macroalgae

heterotrophs light

bacteria oxygen

nutrient limitation

competition

control

continental / terrestrial influenceC) terrestrial

PmaxRiver Deltas sewage outfalls

influence

Caribbean sea

Gulf of Mexico

oceanic reefsD) modern

upwelling zones equatorial meridional

settings

subtropical

coral reef turn-on/turn off zone

Fig. 1.14. Nutrient concentration in surface waters of the ocean and its effect on marine biota. Nutrient levels are expressed in milligrams of chlorophyll per cubic meter of seawater on the horizontal axis. On the vertical, various changes induced by changing nutrient levels: A) Dominant benthic organisms. B) Type of limitation imposed on benthic communities. C) Level of nutrient inux from land (with characteristic examples). D) Modern oceanographic settings. After Hallock (2001), modied.

Ic

Ikirradiance

Coriolis effects. In addition to marine nutrients, input of organic matter from land may cause high productivity in the nearshore zone. Ecologists have subdivided the continuum of nutrient levels in the ocean in discrete categories. Fig. 1.14 presents a classication (Hallock, 2001; Mutti and Hallock, 2003) that subdivides the trophic continuum in four categories. Three of them oligotrophic, mesotrophic and eutrophic environments occur on a regional scale in the ocean; they correspond to subtropical gyres, equatorial divergence and coastal upwelling zones respectively. The hypertrophic category represents an extreme situation that may occur locally in the ocean. Benthic organisms and carbonate sediments secreted by benthos are sensitive indicators of marine nutrient levels as shown in Fig. 1.14. One important message is that the preferred habitats of carbonate-secreting benthos are in oligotrophic and mesotrophic environments. Coral reefs, in particular, thrive in submarine deserts and are easily killed by excess nutrients. Life and light

Fig. 1.15. Irradiance (light energy ux) and rate of photosynthesis (red). Two important levels of irradiance have been dened: Ic is the level at which oxygen consumption by respiration equals oxygen production by photosynthesis; Ic denes the lower limit of the euphotic zone. Ik denes the lower limit of the zone of light saturation. Below this level light is a growth-limiting factor. Pmax = maximum photosynthetic rate of the system. After F. Vijn and H. Boscher (written comm.)

The zone of light saturation is the uppermost water layer where light is so abundant that it is not a growth-limiting factor. The thickness of this zone is different for different organisms. The euphotic zone is the water layer where the rate of oxygen production by photosynthesis is larger or equal to the rate of oxygen consumption by respiration. The euphotic zone is the site of most benthic carbonate production and therefore of prime importance for carbonate sedimentology. In modern oceans, the base of the euphotic zone lies between 30 m and over 150 m. Fig. 1.9 shows that the thickness of the euphotic zone varies in response to ocean productivity, river discharge and latitude. (For application of these concepts to carbonate sedimentology see chapter 2). Species diversity and environment

The number of species in an ecosystem is a fundamental question that lacks a simple answer and an evaluation is beLight is a precious commodity for life in the ocean be- yond the scope of this book (see Townsend et al., 2003 for a cause, most marine food chains start with photosynthesis. succinct overview). Here we shall discuss only one example Sunlight is progressively absorbed as it travels through the of diversity variation diversity as a function of salinity. It water column. The sunlit part of the ocean, the photic zone, is particularly relevant for carbonate sedimentology. Fig. 1.16 shows that species diversity has two maxima, therefore occupies the uppermost part of the water column. The transition to the aphotic part of the ocean is very grad- one at normal-marine salinities, the other in fresh water. Diual but marine ecologists have dened two light levels of versity is low in brackish and hypersaline environments. It

P Respiration

photosynthesis

CHAPTER 1: ESSENTIALS OF NEIGHBORING DISCIPLINES

11

50

100

200 water salinity

300

Fig. 1.16. Salinity effects. Diversity of aquatic fauna and ora plotted against water salinity. Diversity peaks in the two most stable environments fresh water and open-ocean sea water. Restricted marine environments are characterized by large variability of salinity. This reduces species diversity. Temperature variations have similar effects and tend to occur together with salinity variations. After Remane and Schlieper (1971), modied.

is probably more than a coincidence that the diversity maxima correspond to two salinity ranges that are commonly available at the Earths surface: fresh water is continuously supplied by condensation of water vapor from the atmosphere and normal sea water is present in large quantities in the ocean. Sea-water composition changes through geochemical cycling but the rate of change is very slow compared to the rates of biotic speciation and extinction (see chapter 5). Regardless of the reason for the salinity-diversity relationship in Fig. 1.16, the pattern provides an excellent tool for determining the degree of restriction of carbonate depositional environments, i.e. the extent to which the environment was cut off from exchange with the open sea (see chapter 2). Paleontologists have collected large data sets on the number of fossil species in sedimentary rocks and their change through geologic time. After correction for preservation effects, these data provide a basis for estimating biodiversity of past environments.

relative number of species

From oceanography , chemistry and biology to geology

The oceanographic, chemical and biological concepts presented above were derived from the study of the modern world, typically based on observations at time scales of seconds to hundreds of years. Study of the geologic record requires expanding the scope to scales of thousands of years to hundreds of millions of years. The principles of this chapter remain valid but new processes come into view that operate at these expanded time scales. The most important ones are biotic evolution, plate tectonics, and geochemical cycling, i.e. the chemical fractionation and recombination of materials as a result of mantle convection and plate tectonics. The kind of interaction between these long-term processes and the principles just described may be illustrated by a few examples.

Ecologic concepts such as logistic growth and r-K life histories were certainly valid in the past. The difculty is to properly apply them to organisms that differ from their modern counterparts because of evolutionary change. For instance, the question of which invertebrate carbonate producers of the past had photosynthetic symbionts is a matter of intense debate among paleontologists. The answers are highly relevant for modeling carbonate production of the past (see chapter 2). A similar situation holds for oceanography. We have no reason to doubt that general principles of oceanic layering and circulation, such as mixed layer, thermocline, Ekman transport etc. were valid in the past. It is also virtually inevitable that surface circulation was always largely governed by wind shear and deep circulation by temperaturesalinity related density differences. This notwithstanding, the circulation patterns of past oceans differed considerably from the modern situation. One reason is that plate tectonics continually changed the shapes and positions of ocean basins and continents. For instance, the globe-circling current around Antarctica developed in the Oligocene when tectonic movements opened critical seaways. On the other hand, Cenozoic closure of the seaways between North and South America, and Europe and Africa blocked the globecircling equatorial current that had existed in the Late Jurassic and Cretaceous. Another reason for major changes in oceanic circulation may be the conditions at the poles. For instance, Hay et al. (2004) questioned that the large gyres in the surface ocean could form under reduced temperature gradients such as in the Mid-Cretaceous. They suggested an alternative model of surface circulation by ephemeral, migrating eddies. Finally, ocean chemistry as well as the mineralogy of carbonate skeletons varied in the past, mainly because of changing rates of chemical cycling through the crust and the mantle and evolutionary changes of biota at the Earths surface. Chapter 5 discusses examples of such oscillations that directly affected carbonate sedimentation.

C HAPTER 2

Principles of carbonate productionThree rules capture the peculiar nature of carbonate depoc.1 controlled precipitates by photo-autotrophic orsitional systems carbonate sediments are largely of organic ganisms that generate organic matter from disorigin, they can form wave-resistant structures and they are solved substances and sunlight, and easily altered by diagenesis because the original minerals c.2 controlled precipitates by heterotrophic organisms are metastable. The implications of these rules are pervathat are independent of light but require particusive. We will encounter them throughout the chapters of late organic matter for food. this book, starting with the present review of principles that Currently, organisms have the rst hand on precipitation in govern the production of sediments and the growth of reefs. most carbonate settings and abiotic precipitation will kick in if biotic xation is insufcient. Thus, abiotic precipitation is a sort of default setting in the carbonate system of modern MODES OF MARINE CARBONATE PRECIPITATION oceans (term credit to Ron Perkins). Precipitation of solid matter from the dissolved load of the sea occurs either abiotically, governed by inorganic thermodynamics and reaction kinetics, or biotically as a consequence of the metabolism of plants and animals. Precipitation of marine evaporites is an example of an abiotic process, precipitation of marine opal by diatoms or radiolaria an example of a biotic one. Marine carbonate precipitation proceeds along abiotic and biotic pathways and this makes it particularly diverse and complex. The interplay of abiotic and biotic processes during four billion years of organic evolution and environmental change has led to a stunning diversity of precipitation mechanisms that are far beyond the scope of this book. However, even if one focuses on the practically relevant aspects of carbonate precipitation, the subdivision into abiotic and biotic precipitation is inadequate. Work in the past two decades in particular has shown that it is sedimentologically very advantageous to further subdivide the biotic category. I follow Lowenstam and Weiner (1989) who recognized three degrees of biotic inuence on precipitation in general and on carbonates in particular: (Fig. 2.1) a. Abiotic (or quasi-abiotic) precipitates where biotic effects are negligible. b. Biotically induced precipitates where the organism sets the precipitation process in motion but organic inuence on its course is marginal or absent. The reaction takes place outside the cell and the product is very similar, often indistinguishable, from abiotic precipitates. c. Biotically controlled precipitates where the organism determines location, beginninng and end of the process, and commonly also composition and crystallography of the mineral. All skeletal carbonate falls in this category. From an environmental perspective, it is important to further subdivide skeletal carbonates into:

MARINE CARBONATE PRECIPITATION

abiotic ("default setting")

biotic

biotically induced (organomineralic + biomineralic, mainly microbial, precipitates)

biotically controlled (skeletal)

heterotrophic

autotrophic (mainly photosynthetic)

Fig. 2.1. Pathways of carbonate precipitation in aquatic environments a cascade of options governed by the degree of biotic inuence. After Schlager (2000), modied.

Abiotic marine carbonate precipitation Organisms and organic matter are so common in depositional environments and have so many ways of inuencing carbonate precipitation that it is virtually impossible to demonstrate that a particular carbonate precipitate from a natural environment is totally abiotic. However, there are carbonate materials for which the organic inuence, if present at all, is very subtle. Their texture and mineralogy can be reproduced abiotically in the laboratory and their natural occurrence is governed by rst-order trends in ocean chemistry. These materials are included here.

13

14

WOLFGANG SCHLAGER

The most conspicuous abiotic precipitate is cement formed in the pore space during the early stages of diagenesis when the deposit was still in the depositional environment. Burial cements are excluded from the abiotic carbonate factory because they are not derived from sea water but largely from remobilized sedimentary material. The case for abiotic origin is particularly strong for acicular aragonite cements. Acicular magnesian calcites may be biotically inuenced (Morse and Mackenzie, 1990). Associated with tropical skeletal carbonate we nd abiotic precipitates in the form of ooids. Ooids form in highenergy environments by stepwise accretion on a nucleus. Field observations and laboratory experiments indicate a growth history of alternating phases of accretion and rest. The degree of organic inuence on the precipitation process remains a matter of debate. However, two arguments tip the balance in favor of abiotic precipitation: (1) growth of aragonitic, Bahama-type, ooids in the laboratory where the precipitation was essentially abiotic and organic matter had only a modulating effect (Davies et al., 1978) and (2) the similarity of ooids and cements in terms of mineralogy and chemical signature (Morse and Mackenzie, 1990). It seems that the role of organisms and organic matter in the formation of ooids is not sufcient to signicantly alter the abiotic controls (Morse and Mackenzie, 1990; see Reitner et al., 1997 for contrasting view on Great-Salt-Lake ooids). The origin of carbonate in whitings, clouds of carbonate suspended in sea water, is a much debated issue. Morse and Mackenzie (1990) conclude that abiotic precipitation (probably on nuclei of suspended sediment) is very likely for Bahamian whitings. However, in-situ experiments by Yates and Robbins (1999) strongly suggest that blooms of unicellular algae trigger the rst precipitation, probably followed by extended abiotic growth of the original, biotically induced precipitates (Yates and Robbins, 1999, p. 135). I consider the carbonate mud from whitings a mixture of biotically induced and abiotic precipitation. The occurrence of acicular cements, ooids and whitings in modern oceans indicates strong control by inorganic marine chemistry of the sea water. In rst approximation, oolites, aragonitic sea-oor cements and whitings occur in the zone of highest carbonate supersaturation in the ocean the mixed layer of the tropical seas. Ooids and whitings are restricted to this zone, acicular cements are most abundant there. In the temperate latitudes, sea-oor cementation is rare and destructive sea-oor diagenesis tends to dominate. The correlation of needle cements, ooids and whitings with sea-water chemistry is very helpful in predicting at least rst-order trends in the geologic record and warrants their classication as principally abiotic precipitates in spite of evidence of some biotic inuence. Biotically controlled precipitation The majority of carbonate material in modern oceans is precipitated as highly structured skeletons of organisms. Precipitation is primarily controlled by the biochemistry

Autotrophic producers Cyanobacteria (only biotically induced precipitates) Coccolithophorid algae (Haptophyceae) Green algae (such as dasycladaceans, codiaceans) Red algae Autotrophic production via symbionts Many larger foraminifers Hermatypic corals (Scleractinia) Certain bivalves (Tridacnids, rudists?)

Heterotrophic producers Foraminifera Archaeocyathans Sponges (e.g. pharetronids, stromatoporoids, chaetetids) Ahermatypic corals (Scleractinia) Most bivalves Gastropods Cephalopods Arthropods (e.g. trilobites, ostracodes, barnacles) Brachiopods Bryozoans Echinoderms

Fig. 2.2. Important autotrophic and heterotrophic carbonate producers.

of the respective organisms (such as algae, foraminifera or corals); the organisms, in turn, are inuenced by the conditions of the sea they live in, particularly light, temperature and water chemistry (for instance the degree of carbonate saturation of the sea water). To appreciate the effects of various environmental factors we need to recall the two fundamental types of metabolism introduced on p. 9. Autotrophic organisms nourish themselves by utilizing inorganic materials to synthesize their own living matter; heterotrophic organisms have to rely on organic material to do so. Autotrophic organisms among the carbonate producers are almost exclusively photo-autotrophic: they perform photosynthesis and thus depend on light for their livelihood. Some carbonate-secreting organisms are themselves heterotrophs but live in symbiosis with autotrophic algae. As a result, the system of host plus symbiont becomes autotrophic. Fig. 2.2 gives an overview of important autotrophic and heterotrophic carbonate producers. Note that the metabolism of extinct groups can only be deduced from circumstantial evidence. Light is arguably the most important control on skeletal carbonate precipitation because of the dominance of photoautotrophic organisms in carbonate production at least in the Cenozoic. Photosynthesis is a complex, and only partly understood process. The basic reaction may be simplied as CO2 + H2 O + solar energy HCHO + O2 where HCHO represents a simple summary formula for organic matter. The formula clearly illustrates the link be-

CHAPTER 2: PRINCIPLES OF CARBONATE PRODUCTIONproduction in % of maximum 50

15

0

100

0

mean low water

zone of light saturation

tween photosynthesis and carbonate chemistry. Photosynthesis extracts CO2 from the sea water, thus increasing its carbonate saturation and facilitating precipitation of carbonate minerals. For the organisms themselves, precipitation of CaCO3 has the added advantage that potentially deleterious Ca2+ ions can be removed from the system and a protective skeleton can be constructed. The link between skeletal carbonate xation, photosynthesis and light explains the decrease of skeletal carbonate production with water depth in tropical environments. Above sea level, carbonate production rapidly drops to zero in the supratidal zone and becomes negative in most terrestrial environments as carbonate material dissolves in rain water and acidic soils. The typical pattern is shown in Fig. 2.3 and Fig. 2.4 whereas Figs. 2.5 and 2.6 show spe-

approximate depth in meters

100

approximate base of photic zone in clear ocean waters

Fig. 2.3. The prole of carbonate production (red) in a tropical setting from terrestrial elevation to subphotic depth. In most terrestrial environments, production is negative as carbonate rocks are being dissolved by rainwater and acidic soils. Maximum production is in the upper part of the photic zone (zone of light saturation), from where it decreases approximatly exponentially with depth.

carbonate production (P), light intensity (I)Ic Ik Pmax Io

light Iz=I0e-kz

water depth (d)

base light saturation

carbonate production Pz=Pmax tanh(Iz/Ik)

base euphotic zone

Fig. 2.4. Change of light intensity and tropical carbonate production with water depth. Light displays a simple exponential decrease with water depth (black curve and equation). The production of organic matter can be related via a hyperbolictangent function to light intensity (red curve and equation). Production shows a shallow zone of light saturation, where light is not a growth-limiting factor, followed by rapid decrease of organic growth with water depth (denitions in Fig. 1.15). In the tropical carbonate factory, organic production can be taken as a good estimate of carbonate production. In tropical environments, the zone of light saturation reaches to about 20 m for corals, the euphotic zone to about 100 m. Iz = light intensity at depth z, Is = light intensity at base of saturated zone, P = organic production (and also a proxy of carbonate production), z = water depth, k = extinction coefcient of light. After Bosscher and Schlager (1992), modied.

40.0

30.0

depth (m) 20.0

10.0

5.0 10.0 growth rate (mm/yr)Fig. 2.5. Predicted and observed values of coral growth vs. depth. Circles: measured growth rates of Caribbean reef coral Montastrea annularis; red curves: growth rates predicted by the light-growth equation of Fig. 2.4 for common values of water turbidity in the Caribbean. After Bosscher and Schlager (1992), modied.

16Indo-Pacific Papua New Guinea Great barrier reef Great barrier reef

WOLFGANG SCHLAGERCaribbeanGrand Cayman

W. Caribbean

gulf of Aqaba

Pacific atolls

Persian Gulf

Philippines

Singapore

Maladives

Coral Sea

Barbados

Bahamas

Bermuda

Curaao

Red sea

Panama

Jamaica

0

50

100

depth (m)

150

active reef growth strongly reduced growth maximum depth

Fig. 2.6. Depth of the euphotic zone in the Indo-Pacic and the Caribbean, constrained by the limits of reef growth. The base is gradational and varies regionally by tens of meters. After Vijn and Bosscher (written communication).

cic data, mainly from reef environments. The growthdepth curve displays a shallow zone of light saturation followed by a zone of rapid decrease and a third, deep zone where growth asymptotically approaches zero. This growth curve can be derived, via a hyperbolic function, from the well-established exponential decrease of light intensity with depth(Fig. 2.4). The growth-depth patterns of most other carbonate-producing organisms are less well known but seem to follow similar trends. Characterization of growth-depth curves requires denition of two parameters the light saturation zone and the euphotic zone. For both exist stringent biological denitions (Fig. 1.15). Geologists are normally not able to measure the required variables and have to resort to proxy indicators. The euphotic zone is dened in the geologic record as the interval where abundant growth of photosynthetic, carbonatesecreting benthos is possible. The zone of light saturation has not been dened in geological terms. Loosely speaking, it is the interval where light has no recognizable control on the rates of growth and calcication of organisms. The growth forms of corals indicate the zone of severe light limitation by a change from massive to platy colonies (Fig. 2.7). In the light-saturated zone, corals indicate the presence of a very turbulent surface layer of the sea by dominance of branching growth forms.

Temperature rivals light in its effect on skeletal carbonate production. Generally, warmer is better, but there exist upper temperature limits for the various carbonate-secreting organisms. Thus, the temperature window of calcifying benthos is different for different organisms. Most hermatypic (i.e. symbiotic) corals function in the range of 20-30 C. The upper temperature boundary sets important limits to carbonate production, particularly in restricted lagoons where temperatures frequently exceed 30 C. The most important effect of temperature, however, is the global zonation of carbonate deposits by latitude (Figs 2.8, 2.9). In spite of what has just been said about the importance of light, the boundary northern and southern limit of coral reefs, and thus the boundary of tropical and cool-water carbonates, in the modern oceans seems to be controlled by winter temperature rather than radiation. This indicates that as one moves poleward in the modern oceans, the temperature limit for hermatypic coral growth is reached before the light limit. In the past, this need not always have been the case. The temperature limit and the light limit may have shifted relative to one another during geologic history. The fairly stable position of 30-35 latitude for the boundary of Phanerozoic tropical carbonates may reect the joint control by temperature and light.

Yucatan

Aldabra

Florida

Belize

Bikini

Tahiti

CHAPTER 2: PRINCIPLES OF CARBONATE PRODUCTION

17

Fig. 2.7. Growth forms of corals reect the environmental changes related to water depth. Example is from the Caribbean. Branching forms dominate the uppermost, high-energy layer of the sea. Domal and massive forms occupy intermediate depths. Below the zone of light saturation, corals become platy and foliose in order to capture a maximum amount of light. At the biologically dened lower limit of the euphotic zone, coral growth in this region is already negligible. Vijn and Bosscher, written communication.

Fig. 2.8. Comparison of terminologies proposed for carbonate sediments of different climatic zones. Most terms are created by combining parts of the names of the most common groups of organisms. Boundary between T and C factories used in this book approximately coincides with the subtropical/temperate boundary of most authors. Based on Mutti and Hallock (2001) and Schlager (2003).

T factory

C factory

18 WOLFGANG SCHLAGER

?

Fig. 2.9. Temperature control on reefs. Recent tropical reefs (red) are limited in the north and south by the position of the 20 C-isotherm for the coldest winter month, shown here as a bold line. Cool-water bioherms (blue) occur almost exclusively pole-ward of this line. The 20 C-isotherm follows the 30 latitude line only approximately. In the eastern parts of the Atlantic and Pacic Oceans the isotherms bend towards the equator because cold water is upwelling there. If radiation were the dominant control on the distribution of reefs, the northern and southern limits of the reef belt should parallel latitude much more closely. Compiled from ReefBase (www.reefbase.org), James (1997) and Van Loon (1984).

CHAPTER 2: PRINCIPLES OF CARBONATE PRODUCTIONHawaii Maui 10 Oahu FFS Maro 5 Nihoa Necker Laysan Lisianski Gardner 23 24 25 26 latitude ( N)reef complexes mixed with bryozoan-algal biostromes

19

reef accretion (kg CaCO /m/y)

15

Darwin Point

P&H Midway Kure 27 28 29

19

20

21

21

massive coral-algal reef complexes

bedded bryozoanalgal bioclastic debris; reef growth during climatic optima only

0

10

20

30

40

Fig. 2.10. Northward decrease of reef growth and change to cool-water carbonate deposition in the North Pacic. Upper panel: decrease of the rate of reef growth with latitude. Darwin Point marks the northern limit of reef growth. Lower panel: latitudinal change from tropical to cool-water carbonate facies observed on the Hawaii-Emperor chain of islands and seamounts. Black dots and dashes carbonate debris, green coral reefs, blue bryozoan biostromes. Compiled using Schlanger (1981) and Grigg (1982).polepole

pole

5033

34 35

50

10

5015 2025

0

5

36

10 15

3037

303536 29

30

20

28

35 0pole pole

25

0

0

50 Foramol 30Chlorozoan

50absent

30Ooid/aggregate

pellet 0 0

Fig. 2.11. Carbonates in cool latitudes and tropical latitudes a comparison. Figs A, B, C illustrate changes in environmental conditions; Fig. D illustrates difference in skeletal carbonate temperate (= cool-water) carbonates are dominated by benthic foraminifers and molluscs (foramol association ), tropical latitudes by green algae and corals (chlorozoan association ); Fig. E shows that nonskeletal grains (ooids, peloids) are virtually absent in temperate-water carbonates. After Lees (1975), modied.

20

WOLFGANG SCHLAGER duced by micro-organisms, mostly bacteria and cyanobacteria. Micrite is a major, often dominant component of these deposits. The term (mud) mound is commonly used as a eld-geologic term (Wilson, 1975; James and Bourque, 1992). For the deposits themselves, the term microbialite is widely used. The drawback is that the word has a strong genetic connotation. If one wishes to avoid this explicit statement on genesis, the term automicrite is recommended. It stands for autochthonous micrite as opposed to allochthonous micrite that was transported and deposited as ne-grained sediment (Wolf, 1965). Whether the micrite formed as a rigid precipitate can often be deduced from thin sections or polished slabs. The past decade brought enormous progress on the orgin of mud mounds and other automicrite deposits. The combination of detailed eld work, petrography and collaboration with biologists and organic chemists has led to detailed insight in a geologically very important carbonate precipitation mode that differs signicantly from the more conspicuous skeletal mode (Monty et al., 1995; Reitner et al., 1995a; 1995b; Neuweiler et al., 2003). The environmental controls on microbial precipitation are less well known than those of skeletal precipitation. An important property of the microbial mode of precipitation is its near-independence of light. Microbial precipitates may form in the photic zone or below, certainly to depths of 400 meters. On modern reefs, the microbial deposits are best developed in the forereef environment. However, stromatolites in the uppermost photic zone (e.g. Reid et al. 2000) and automicrite in the interstices of coral framework (Camoin et al. 1999) demonstrate that the microbial mode of carbonate xation nds its niches even in the prime domains of skeletal production. An important chemical requirement is supply of alkalinity in the form of the anions HCO3 and CO2 . A likely source 3 of alkalinity is sulfate reduction combined with decay of organic matter in oxygen-decient layers of the ocean such as the oxygen minimum of the thermocline. The estimated water depth and organic-rich ambient sediments of many mud mounds support this assumption. Whether temperature sets practically relevant limits for microbial carbonate precipitation is unclear. Mud mounds seem to be best developed in low latitudes. However, the paleo-latitude of many Paleozoic mounds is not well constrained and narrow latitudinal restriction is not to be expected with a production system that demonstrably functions at low light levels and in indermediate water depths, i.e. at temperatures signicantly below tropical surface temperatures.

Latitudinal zonation of skeletal production. Skeletal carbonate production changes very signicantly with latitude. The differentiation into tropical and cool-water carbonates is widely applied and often further subdivided (Figs 2.8, 2.10, 2.11; Lees, 1975; Tucker and Wright, 1990; James and Kendall, 1992; James, 1997). Tropical carbonates are dominated by photosynthetic organisms and usually include metazoan reefs, abundant green algae and larger foraminifera. Cool-water carbonates lack these deposits and consist mainly of skeletal sand and gravel derived from molluscs, bryozoans, smaller foraminers and red algae. The contribution of photo-autotrophs to cool-water carbonate production is limited to red algae that are normally not the dominant component. Consequently, the depth window of cool-water carbonate production is much wider. It should be noted that the zone of tropical carbonates reaches to 30-35 of latitude and thus extends from the humid tropics to the desert belt of the horse latitudes (Fig. 2.9). The cool-water realm extends over several climate zones, reaching from the northern limit of the desert belt to the polar regions (Fig. 2.11). The differences of the tropical and the cool-water realm are not restricted to the skeletal material. Cool-water carbonates also are distinct by the absence of mud, shallow reefs and oolitic sand shoals with early cementation. The lack of reefs and cemented shoals has fundamental implications for the depositional anatomy. Nutrients. Contrary to common expectations, high-nutrient environments are unfavorable for many carbonate systems. Nutrients, to be sure, are essential for all organic growth, including that of carbonate-secreting benthos. However, the carbonate communities dominated by autotrophs, such as reefs, are adapted to life in submarine deserts. They can produce their organic tissue with the aid of sunlight from sea water with very low nutrient levels and are very efcient in recycling nutrients within the system. In highnutrient settings, the carbonate producers are outpaced by soft-bodied competitors such as eshy algae, soft corals or sponges. Furthermore, the destruction of reef framework by bio-erosion increases with increasing nutrient supply. Salinity varies relatively little in the open-marine environment. The effects of these subtle variations on carbonate production are not well known. Where access to the open ocean is restricted, salinity varies greatly and signicantly affects the diversity of the biota (Fig. 1.16). The combined effects of salinity and temperature variations allow one to subdivide carbonate environments (Fig. 2.12). Biotically induced precipitation

In the last two decades, it has been demonstrated that a subdivision of shoal-water carbonates into abiotic and biotPrecipitation modes in comparison ically controlled (skeletal) material is inadequate. A signiThe boundaries of the three precipitation modes are gracant portion of the non-skeletal carbonate material has been precipitated under the inuence of organisms and thus can- dational. The degree of biotic inuence in the induced not be classied as abiotic. Commonly precipitation is in- and controlled categories varies considerably and even the

CHAPTER 2: PRINCIPLES OF CARBONATE PRODUCTION

21

PROTECTION AND RESTRICTION IN CARBONATE ENVIRONMENTS ``` ``` RESTRICTION ``` `` BIOTIC RESTRICTION EVAPORITES PRESENT PROTECTION ` NO RESTRICTIONAGITATED (no mud) CALM OR EPISODICALLY AGITATED (muddy sediments) e.g. skeletal sand shoal White Bank in Florida Reef Tract e.g. deep muddy lagoons of